Field of the Invention
The present invention relates to a method for improving the acoustic sidetone suppression in hands-free telephones, having a level discriminator, frequency-selective, controllable echo cancellation with subband processing and a residual-error post filtering.
In the case of hands-free telephones it is absolutely necessary to suppress the signals of the remote subscriber which are emitted by the loudspeaker and therefore received again by the microphone, since otherwise unpleasant echoes disturb the connection. Until now, it has been customary for the purpose of suppressing these echoes, that is to say for the purpose of acoustic sidetone suppression, to provide a level discriminator which strongly damps the transmitting or receiving path as a function of the call situation. However, this renders duplex (full duplex mode) virtually impossible.
It has already been attempted using the prior art to date to make an adequate sidetone suppression available despite acceptable duplex mode properties. For this purpose, frequency-selective, controllable echo cancellation is provided in addition to the level discriminator. In this regard, reference is made to the as yet unpublished patent application DE 197 14 966 of the applicant. Other methods are described, for example, in the NEC advertising brochure xe2x80x9cRefleXion(trademark) Acoustic Echo Canceller on the xcexcPD7701x Familyxe2x80x9d, 1996, or in the description of the Motorola DSP5600x digital processor (M. Knox, P. Abbott, Cyox: A Highly Integrated H320 Audiosubsystem using the Motorola DSP5600x Digital Processor) . However, given the long signal propagation times of video conference links, and/or in the case of GSM links, these methods also do not offer adequate echo suppression if the aim is simultaneously for a duplex to be possible.
It has therefore already been proposed to provide an additional post-filtering downstream of the frequency-selective echo cancellation with the aid of subband processing. Such a post filtering is known, for example, from the article xe2x80x9cV. Turbin, A. Gilloire, P. Scalart: Comparison Of Three Post-Filtering Algorithms For Residual Acoustic Echo Reductionxe2x80x9d ICASSP97, International Workshop on Acoustic Speech and Signal Processing, Munich 1997, or from the article by R. Martin xe2x80x9cAn improved Echo-shape Algorithm for Acoustic Echo Controlxe2x80x9d, EUSIPCO96, 8th European Signal Processing Conference, Triest, Italy, 1996. So far, these concepts could only be implemented with difficulty, since it is necessary, after all, to provide digital signal processing both for the echo cancellation with the aid of subband processing and for the post filtering, and the computing performance required for this cannot be delivered with acceptable outlay by the processors currently available.
It is therefore the object of the invention to specify a method for improving the acoustic sidetone suppression in hands-free telephones, in the case of which the computational outlay is minimized such that it is possible to implement both a frequency-selective echo cancellation with the aid of subband processing, and the required post filtering on customary xe2x80x9cconsumer processorsxe2x80x9d. This object is achieved with the aid of a method having the features of Patent Claim 1. Advantageous refinements of this method are specified in the subclaims.
According to the invention, therefore, use is made only of a single controlled variable, specifically the step-size vector, both for controlling the frequency-selective echo cancellation, and for controlling the further filter. Use can preferably be made in this case of a plurality of different sampling rates. The computational outlay can be further reduced thereby.
It is likewise preferred to use adaptive filters both for the echo cancellation and for the further filter.
The echo cancellation is preferably implemented by means of a filter bank in frequency subbands.
Preferably both power-based estimates and correlation-based analyses are used for controlling the adaptation or step size.
It is likewise preferred for power transmission factors to be estimated in subbands in order to determine the step size.
It is likewise preferred for both the echo cancellers and the residual-error post filtering to supply the estimated values for the return loss introduced by them, since these estimated values can preferably be used for controlling the attenuation of the level discriminator. Consequently, the attenuation to be introduced by the level discriminator can be further reduced, and thus the call quality can be further improved in the case of duplex.
It is additionally preferred to detect the simultaneous activity of both call participants (duplex). It is then possible, for example, to reduce the overall attenuation of the level discriminator in the case of duplex, in order further to improve the full duplex mode of the hands-free telephone.
The present invention is described in more detail below with the aid of the exemplary embodiment illustrated in the attached drawings, in which: